can I have an iphone PC backup within dropbox?

For me, a backup to iCloud is an easier way to accomplish that, plus it happens automatically when the phone is plugged in to charge while connected to wi-fi. You can turn it on in Settings > iCloud > Backup. If you don't want to do that, then you could copy your iTunes backup folder to Dropbox but there isn't any option in iTunes to change the normal location. See the quote below from Find and manage your iTunes backups - Apple Support

iTunes saves backups to your user folder. The location of this folder varies depending on your operating system, and the location can't be changed.

If you want to have extra copies of your iTunes backups, you can copy the backup folders to another location, external drive, or network storage location.

No -- the Settings > iCloud > Backup is on your phone (i.e., the Settings app) -- although I think you can also turn it on by connecting the phone to iTunes and doing it there,

There are some small differences. For Contacts turn on syncing (Settings > Cloud > Contacts). For details on the differences see the information in the quote below from About backups in iCloud and iTunes - Apple Support and there are some more details in iCloud: iCloud storage and backup overview

Also note that whether you use iTunes or iCloud backup, if you have Photos on your phone that are important to you, it is a good idea to have them somewhere you can actually visually verify that you have a copy. That can be done by importing them to a computer via a USB cable, by using Photo Stream to send them to a second device or place, by using 3rd party software to send them to a computer (e.g. http://www.photosync-app.com/), by turning on Settings > iCloud > Photos > iCloud Photo Library, or by using a 3rd party such as Dropbox or Flickr.

iCloud backups

With a Wi-Fi network connection, you can make a backup of your device using iCloud. You don’t need to plug your device into a computer or even be at home to make a backup with iCloud.

iCloud backups include nearly all data and settings stored on your device. iCloud backups don't include:

• Data that's already stored in the cloud, like contacts, calendars, notes, My Photo Stream, and iCloud Photo Library
• Data stored in other cloud services, like Gmail and Exchange mail
• Apple Pay information and settings
• Touch ID settings
• Content you didn't get directly from iTunes, App, or iBooks Stores, like imported MP3s, videos, or CDs
• iTunes in the Cloud and App Store content
  If it's still available in the iTunes, App, or iBooks Store, you can tap to re-download your already purchased content.

Learn how to manage your iCloud storage or delete iCloud backups that you no longer need.

iTunes backups

From your Mac or PC, you can make a backup of your device in iTunes. Syncing your device with your computer isn't the same as making a backup.An iTunes backup includes nearly all of your device's data and settings. An iTunes backup doesn't include:

• Content from the iTunes and App Stores, or PDFs downloaded directly to iBooks
  You can back up this content using Transfer Purchases in iTunes.
• Content synced from iTunes, like imported MP3s or CDs, videos, books, and photos
• Photos already stored in the cloud, like My Photo Stream, and iCloud Photo Library
• Touch ID settings
• Apple Pay information and settings
• Health and Keychain data
  To back up Health and Keychain data, you'll need to use Encrypted Backup in iTunes.